Step 1: Identify Megasthenes:
Megasthenes was a Greek diplomat and historian sent as an ambassador by the Seleucid king Seleucus I Nicator to the Mauryan court of Chandragupta Maurya at Pataliputra.
Step 2: State when he came:
He arrived in India around 302 BCE, following the 303 BCE peace treaty between Seleucus and Chandragupta, and resided at the Mauryan court for several years, later writing the account 'Indica' describing Mauryan society and administration.
Final Answer:
\[ \boxed{\text{Megasthenes: Greek ambassador of Seleucus I at Chandragupta Maurya's court, arrived c. 302 BCE}} \]